The present invention relates to hydraulic apparatus, and in particular to a hydraulic power assist for a manual steering system for an automotive vehicle.
Normally, in an automotive vehicle, the vehicle is steered by a manual interconnection between a hand wheel, i.e., steering wheel and the steerable wheels of the vehicle. The interconnection includes a gear reduction or steering gear set. A hydraulic system is coupled to a steering gear to assist the operator by lessening the manual effort required to steer the wheels.
A typical hydraulic power steering assist system includes a power steering pump and a power steering motor. The motor is connected to the manual steering gear and provides the assistance to the vehicle operator's manual steering effort. The pump is driven by the vehicle engine. A valve is interposed between the pump and power steering motor. The valve controls fluid flow from the pump to the power steering motor. The valve responds to turning of the vehicle steering wheel. The valve functions to control (i) the direction of fluid flow to the steering motor and (ii) the magnitude of the flow to achieve the desired amount of steering assist.
The vehicle steering maximum steering efforts usually occur at low vehicle speeds. Hence, the pump is required to deliver maximum flow and pressure at low engine speeds.
The pumps which are used in power steering systems normally provide an increasing output flow as pump speed increases. Some pumps have increasing flow up to a predetermined speed, and thereafter provide a substantially constant output flow at pump speeds above the predetermined speed. Such flow characteristics are achieved by different pump constructions. Typical pump constructions are shown in U.S. Pat. Nos. 4,014,630; 3,817,266; and 4,516,918. A certain amount of energy is used to drive each pump. Clearly, it is desirable to provide a steering system in which less energy is used for driving the power steering pump.
Moreover, there has been an increasing interest in using electronic elements in hydraulic power assisted steering systems. Known systems commonly include an electrically actuated valve which controls both direction of fluid flow and magnitude of fluid flow to the steering motor. Such a valve must operate at high flow and high pressure and consequently has substantial fluid forces acting thereon. As a result, a solenoid or other means for moving the valve must be relatively large, thus having high power requirements.
Also, under certain operating conditions, the power steering pump may be required to supply fluid under high pressure. Sometimes, the pressure developed by the pump becomes excessively high. For example, when the wheels of the vehicle encounter a large resistance to turning. This results in an increased engine load which tends to stall a small engine. Pressure limiting valves are used to avoid such high pressures being generated by the pump.